[FIX] test_mail: prepare and fixup tests for alias domains

Some tests are updated to lessen diff in future tests, especially about
aliases. Some tests are fixed as they are somehow incorrect (notably
in gateway testing) but currently passing as mail gateway is quite
permissive.

Add some tests preparing MC / alias domains configuration notably about
company / alias synchronization, which is currently only based on config
parameters.

Also update some tests by using fstrings which are generally more readable.

Finally move some tests to their right file / main testing class to keep
them ordered by main topic.

Prepares Task-36879 (Mail: Support MultiCompany Aliases)

closes odoo/odoo#136102

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>